The present invention relates to a method of forming an insulation film on the surface of a semiconductor device. More particularly, the invention relates to a method of forming a stable and dense insulation film on the surface of a semiconductor device, especially an integrated circuit, or IC.
In general, in order to minimize aging variation of the characteristics and improve reliability in manufacture of a semiconductor device, an area where the pn junction is exposed on the surface of said device is often covered with an insulating film. Furthermore, in a MISFET and MIS IC, a gate insulating film is provided in order to induce an inverting layer channel. Particular stability of the characteristics is therefore required. Usually, the semiconductor is silicon, the insulating film is silicon dioxide and the metal is aluminum.
However, the aforedescribed structure has the disadvantage that the gate threshold voltage varies greatly when an electrical field of about 10.sup.6 V/cm is applied to the insulating film at a temperature of 200.degree. C. This phenomenon seems to occur as a result of variation of space charge distribution at the interface of the silicon and the insulating film due to the presence of trapping levels of the carrier caused by structural defects of SiO.sub.2 and the drift of impurity ions migrating into the SiO.sub.2. It is obvious that the effect of such variation of space charge due to structural defects at the interface of the Si and insulating film becomes most distinctive. For this reason, various coating techniques, such as vapor growth and sputtering etc., have been investigated instead of thermal oxide SiO.sub.2 which easily permits the mingling of ions, such as the alkali ion. However, defects at the interface cannot be reduced more than those of the thermal oxide SiO.sub.2.
On the other hand, thermal oxide SiO.sub.2 produced in a very clean ambient is maintained at a defect of 10.sup.11 /cm.sup.2 in terms of the surface charge density. However, there is still a defect, such as excessive silicon ion etc., at the interface. This is a serious problem.
The principal object of the invention is to provide a method of forming an insulating film on the surface of a semiconductor device, which method overcomes the disadvantages of known methods.
An object of the invention is to provide a method of forming a high density silicon nitride film on the surface of a semiconductor substrate.
Another object of the invention is to provide a method of forming a homogeneous high density silicon nitride film on the surface of a silicon substrate.
Still another object of the invention is to provide a method of forming a comparatively thick, high density, homogeneous silicon nitride film on the surface of a silicon substrate.